The present invention relates to a radiation detection apparatus and a radiological imaging apparatus, and in particular, to radiation detection apparatus in which semiconductor radiation detection elements can be replaced with new ones, and a radiological imaging apparatus such as a single photon emission tomography apparatus (hereinafter referred to as a SPECT apparatus) and a positron emission tomography apparatus (hereinafter referred to as a PET apparatus which uses the radiation detection apparatus.
Conventional radiation detectors detecting radiation such as a γ ray are known to use an NaI scintillator (JP-A-2000-241555 (paragraph 0019, FIG. 1)). In a gamma camera (a kind of radiological imaging apparatus) comprising an NaI scintillator, radiation (γ ray) is incident on the scintillator at an angle limited by a large number of collimators. The radiation thus interacts with an NaI crystal to emit scintillation light. The light reaches a photomultiplier via a light guide to become an electric signal. A measuring circuit mounted on a measuring circuit fixed board shapes the electric signal and sends it to an external data collecting system from an output connector. The scintillator, light guide, photomultiplier, measuring circuit, measuring circuit fixed board, and the like are wholly housed in a light blocking shield case to block electromagnetic waves other than external radiation.
In general, a gamma camera using a scintillator has a large photomultiplier placed behind a large crystal such as NaI. Accordingly, a positional resolution is limited to the order of several to over 10 mm. Further, the scintillator carries out detection through many conversions including one of radiation into visible light and one of visible light into electrons. Consequently, the scintillator disadvantageously has a low energy resolution and cannot resolve entering scattered radiation. This results in a decrease in S/N ratio for a signal representative of real positional information on the emission of a γ ray. Accordingly, problems with the scintillator include degraded images or an increase in the time required for imaging. Some PET apparatuses (Positron Emission Tomography apparatuses) have a positional resolution of 5 to 6 mm and some high-end PET apparatuses have a positional resolution of about 4 mm. However, they also have a problem attributed to the S/N ratio.
A radiation detector detecting radiation on the basis of a principle different from that of the scintillator is a semiconductor detector comprising semiconductor radiation detection elements composed of a semiconductor material such as CdTe (cadmium telluride), TlBr (thallium bromide), GaAs (gallium arsenide) (see JP-A-2000-241555 (Paragraph 0019, FIG. 1) and JP-A-7-50428 (Page 2, FIG. 3).
In the semiconductor detector, the semiconductor radiation detection elements convert charges resulting from the interaction between radiation and the semiconductor material, directly into an electric signal. Accordingly, the semiconductor detector can accomplish conversions into electric signals more efficiently than the scintillator and has an excellent energy resolution. Much attention is thus being paid to the semiconductor detector. In this case, the excellent energy resolution means an increase in the S/N ratio of a radiation detection signal indicative of the real position information, that is, an improvement in detection accuracy. Various effects are thus expected such as an improvement in image contrast and a decrease in the time required for image pickup. By two-dimensionally arranging the semiconductor radiation detection elements on a substrate, it is possible to detect the position of an emission source of radiation.
To improve the sensitivity and energy resolution of a semiconductor detector constructed by two-dimensionally arranging semiconductor radiation detection elements, it is necessary to densely arrange the semiconductor radiation detection elements on the substrate so as to minimize dead spaces and capture incident radiation without leakage. However, mounting members or portions must be provided which are used to mount a semiconductor detector in an apparatus, the semiconductor detector having a large area in which a large number of, for example, one hundred thousand semiconductor radiation detection elements are two dimensionally arranged. In this case, dead spaces may be created in order to install the mounting members or portions or areas may be created in which no semiconductor radiation detection elements are arranged. For example, as shown in FIG. 8A, a semiconductor detector 64 is known in which a detector substrate 61 having a large number of semiconductor radiation detection elements 62 two-dimensionally arranged on one surface and a signal reading circuit mounted on the other surface is installed on a circuit board 63 using female connectors 63a fitted around male connectors 61a provided on the detector substrate 61. As shown in the bottom view in FIG. 8B, the semiconductor detector 64 requires a space S1 used to allow mounting jigs such as bolts 65a to be installed at an end of the detector substrate 61 in order to mount each semiconductor detector 64 in a radiation detection section of a radiological imaging apparatus. No semiconductor radiation detection elements can be arranged in the space S1. The presence of the space S1 prevents the improvement of sensitivity of the semiconductor detector 61 based on the dense arrangement of the semiconductor radiation detection elements. Further, in the semiconductor detector 61, shown in FIGS. 8A and 8B, if the radiological detection performance of the semiconductor detector is degraded as a result of failures in or damage to some semiconductor radiation detection elements, the whole semiconductor detector must be replaced with a new one or the defective semiconductor radiation detection elements must be removed and separated from the may other semiconductor radiation detection elements installed. Thus, as shown in the bottom view in FIG. 9, a large semiconductor detector may be constructed by producing substrate units 71a, 71b, 71c, and 71d on which semiconductor radiation detection elements are installed and mounting each of the units 71a, 71b, 71c, and 71d on a fixing substrate (not shown). However, even the semiconductor detector shown in FIG. 9 requires a space S2 used to allow the installation of mounting jigs 72 for mounting each of the units 71a, 71b, 71c, and 71d on the fixing substrate. Consequently, no semiconductor radiation detection elements can be arranged in the space S2, thus preventing the semiconductor radiation detection element from being densely arranged and the improvement of sensitivity and the spatial resolution of the semiconductor detector.